A battery pack is widely used as a power source, such as, a power tool, an electric assist bicycle, an electric auto-bike, a hybrid vehicle, an electric vehicle, and the like, and in addition as electric power storages for home use, shop use, and the like. In these uses, the battery pack having the waterproof structure, can be used outdoor at ease. As the battery pack having the waterproof structure, the following structure is developed. The battery core pack is inserted in the waterproof bag, and this is stored in an outer case. (refer to Patent Literature 1)